In its simplest form, automotive speed control systems utilize a tachometer generator which provides an output voltage which is converted into square wave pulses of constant, predetermined pulse duration, for example by a monostable multivibrator. An integrator, for example an R/C network, then determines the average value, providing a d-c output signal the amplitude of which is proportional to instantaneous vehicle speed. When the vehicle has reached the speed which is desired to be maintained, then a switch is operated and the then instantaneously appearing d-c voltage is stored in a capacitor. Upon charge of the capacitor to the voltage at the commanded speed, a command signal is available for later comparison with the actual speed signal. The voltage at the capacitor is sensed by means of a semiconductor element having a high resistance input, for further processing in the control system. Due to the unavoidable leakage currents as well as the loading of the semiconductor element -- even though it may have a very high resistance input -- the capacitor voltage will decrease. As a result, the commanded speed cannot be maintained constant over a long period of time by such a system.
It has also been proposed to utilize digital systems in which the square wave signals derived from the tachometer generator in a given time interval are summed in a counter so that the counter state will be a measure for the desired vehicle speed. Such system, however, include stray noise or other interference pulses in the count state so that the command speed number, as stored in the counter, may not conform to the actually desired command speed.